pubs.acs.org/Langmuir © 2010 American Chemical Society
Sticking Polydisperse Hydrophobic Magnetite Nanoparticles to Lipid Membranes Michael Paulus,*,† Patrick Degen,‡ Thorsten Brenner,† Sebastian Tiemeyer,† Bernd Struth,§ Metin Tolan,† and Heinz Rehage‡ ‡
† TU Dortmund, Fakult€ at Physik/DELTA, Maria Goeppert Mayer Strasse 2, 44227 Dortmund, Germany, TU Dortmund, Fakult€ at Chemie, Otto Hahn Strasse 6, 44227 Dortmund, Germany, and §Deutsches Elektronen Synchrotron (HASYLAB), Notkestrasse 85, 22607 Hamburg, Germany
Received July 20, 2010. Revised Manuscript Received September 9, 2010 The formation of a layer of hydrophobic magnetite (Fe3O4) nanoparticles stabilized by lauric acid is analyzed by in situ X-ray reflectivity measurements. The data analysis shows that the nanoparticles partially disperse their hydrophobic coating. Consequently, a Langmuir layer was formed by lauric acid molecules that can be compressed into an untilted condensed phase. A majority of the nanoparticles are attached to the Langmuir film integrating lauric acid residue on their surface into the Langmuir film. Hence, the particles at the liquid-gas interface can be identified as so-called Janus beads, which are amphiphilic solids having two sides with different functionality.
Introduction Because of their multifarious applicability in science and technology, interest in the properties of nanoparticles has grown in the last few years. Their application covers a broad range (e.g., the production of cosmetics, surface coatings, medicine, and optoelectronic applications1-5). Thus, although this class of materials is present in daily life, there is still a lack of information about the impact of nanoparticles on the human organism (e.g., the interaction of nanoparticles with cell membranes). Furthermore, the controlled manipulation of nanoparticles such as the synthesis of nanoparticle layers for surface coatings is important for the production of advanced materials. For such a purpose, the use of nanoparticles with different functionality on different sides (e.g., different surface charges or different hydrophobicity) might open the way to controlling the surface properties of advanced functionalized material. The amphiphilic character of small particles is realized by so-called Janus particles or Janus beads, which offer at least two different sides of physical functionality.6-9 In this work the arrangement of hydrophobic magnetite nanoparticles stabilized by lauric acid at the water-gas interface is investigated. Lauric acid binds only weakly to the nanoparticles. Consequently, it is possible that the hydrophobic shell partially dissolves when the objects are in contact with water. It is shown that this effect can be used to create well-ordered Langmuir films with nanoparticles attached below. This feature might open a new path to the production of Janus beads offering a hydrophobic and a hydrophilic side. *To whom correspondence should be addressed. E-mail: michael.paulus@ uni-dortmund.de. (1) Patil, V.; Mayya, K. S.; Pradhan, S.; Sastry, M. J. Am. Chem. Soc. 2009, 119, 9281. (2) Berkovskyl, B.; Bashtovoy, V. Magnetic Fluids and Applications Handbook; Begell House: New York, 1996. (3) Gupta, A. K.; Gupta, M. Biomaterials 2005, 26, 3995–4021. (4) Pankhurst, Q.; Connolly, J.; Jones, S.; Dobson, J. J. Phys. D: Appl. Phys. 2003, 36, R167. (5) Brust, M.; Kiely, C. Colloids Surf., A 2002, 202, 175–186. (6) Walther, A.; M€uller, A. H. E. Soft Matter 2008, 4, 663–668. (7) Nie, Z.; Li, W.; Seo, M.; Xu, S.; Kumacheva, E. J. Am. Chem. Soc. 2006, 128, 9408–9412. (8) Casagrande, C.; Fabre, P.; Raphael, E.; Veyssie, M. Europhys. Lett. 1989, 9, 251–255. (9) Pradhan, S.; Xu, L. P.; Chen, S. W. Adv. Funct. Mater. 2007, 17, 2385.
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The systems were studied by means of X-ray reflectivity measurements. This technique yields information on the vertical structure of the investigated sample with angstrom resolution. A detailed description can be found in refs 10-12. By applying this method, it was possible to analyze the size distribution of the adsorbed nanoparticles at the liquid-gas interface.
Sample and Experiment Magnetite (Fe3O4) nanoparticles were prepared as described by Meldrum.13 Solutions of 3.0 g of FeCl3 3 4H2O in 12.5 mL of water and 6.0 g of FeCl2 3 6H2O in 12.5 mL of water were combined, and 12.5 mL of ammonium hydroxide (14.8 M) was added under vigorous stirring. The resulting magnetite precipitate was settled by placing a magnet below the beaker, and the supernatant solution was then decanted. Afterwards, the solid was resuspended in 25.0 mL of 0.75 M ammonium hydroxide. The nanoparticles were then isolated again using magnetic separation and decantation. By evaporation in vacuum at room temperature, the remaining water was removed. A solution of 0.4 g of lauric acid in 20.0 mL of ethanol was subsequently added to the solid, and the magnetite dispersion was induced by sonication. The ethanol was removed by rotary evaporation, and the dried magnetite was resolved in 20.0 mL of chloroform using sonication again. After the subsequent extraction, a dry powder of magnetite was obtained. This powder (in 0.5 g quantities) was dissolved in 10.0 mL aliquots of t-butylmethylether (TBME, purity 99þ%, Sigma-Aldrich, Germany). The magnetite suspension was sonicated again, and any insoluble material was separated by centrifugation. The resulting clear, brown suspension was stable and used as the stock solution (29 g/L solid matter). The particles were stabilized by the lauric acid (C12H24O2, purity 99þ%, Sigma-Aldrich, Germany) and had a mean radius of approximatly 4.5 nm, as determined by dynamic light scattering (DLS). The X-ray reflectivity measurements were carried out at beamline BW1 of HASYLAB, Hamburg, Germany, using the liquid (10) Ocko, B.; Wu, X.; Sirota, E.; Sinha, S.; M., D. Phys. Rev. Lett. 1994, 72, 242. (11) Sinha, S.; Sirota, E.; Garoff, S.; Stanley, H. Phys. Rev. B 1988, 38, 2297– 2311. (12) Tolan, M. X-ray Scattering from Soft Matter Thin Films; Springer: Berlin, 1999. (13) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. J. Phys. Chem. 1994, 98, 4506–4510.
Published on Web 09/27/2010
DOI: 10.1021/la102882j
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Paulus et al.
Figure 1. (Left) Reflectivity normalized by the Fresnel reflectivity of water (b), nanoparticles on a water subphase at a surface pressure of π = 0 mN/m (9), and nanoparticles on a water subphase at a surface pressure of π = 6 mN/m (2). For better visibility, the curves are shifted vertically. The refinements to the data are shown by solid lines. (Right) Corresponding electron density profiles obtained by the refinement of the reflectivity data. For better visibility, the profiles are shifted in the horizontal direction. (Dashed line) Gas-water interface; (solid black line) nanoparticles on the water subphase, π = 0 mN/m, and (solid gray line) nanoparticles on the water subphase, π = 6 mN/m. surface scattering setup.14 The samples were prepared in a Langmuir trough that was placed in a water-saturated helium atmosphere to minimize the scattering background and sample contamination. A photon energy of 9.5 keV was used. One reflectivity scan, including the measurement of the diffusely scattered signal, took approximately 20 min. The diffusely scattered signal was subtracted from the reflectivity data in order to obtain the true specular component. First, a reference reflectivity of a bare watergas interface was recorded. Subsequently, 500 μL of lauric acidstabilized Fe3O4 nanoparticles in TBME (c = 0.07 g/L) was spread on the surface. After a waiting time of 20 min and flushing with helium, the reflectivity of the uncompressed layer was recorded. Subsequently, the film was compressed up to a surface pressure of π = 6 mN/m. At this pressure, the reflectivity from the compressed layer was recorded. For the examination of the lateral structure of the nanoparticle arrangement, the film was transferred at a surface pressure of π = 5 mN/m onto a silicon wafer using a dipping process. Subsequently, the prepared film was analyzed by atomic force microscopy (AFM) using a Digital Nanoscope IV instrument.15 An image of the lateral nanoparticle distribution was taken at room temperature. An area of 10 10 μm2 was analyzed.
Data Analysis and Discussion The X-ray reflectivities, normalized by the Fresnel reflectivity, of the water phase and the nanoparticle-covered water subphase taken at different surface pressures are shown in Figure 1. All reflectivities were analyzed using the effective density model12 by applying the Parratt algorithm.16 Data analysis was done in several steps. First, the reflectivity of the bare water surface was refined using a simple model of one liquid-gas interface yielding a surface roughness of σ = (2.9 ( 0.2) A˚. This is in agreement with capillary wave theory assuming a clean water surface.17 In a second step, the reflectivity of the nanoparticle-covered water surface at π = 0 mN/m was refined using a simple model of one layer with a thickness of dπ=0 = (13.9 ( 0.2) A˚ and a surface roughness of σπ=0 = (4.9 ( 0.4) A˚. A possible contribution of the Fe3O4 nanoparticles to the electron density that might be suggested by the low-amplitude oscillations in the reflectivity below qz = 0.1 A˚-1 is not significant. However, at this low surface pressure the packing density of the nanoparticle film might be too (14) Frahm, R.; Weigelt, J.; Meyer, G.; Materlik, G. Rev. Sci. Instrum. 1995, 66, 1677. (15) Degen, P.; Paulus, M.; Leick, S.; Tolan, M.; Rehage, H. Colloid Polym. Sci. 2010, 288, 643. (16) Parratt, L. Phys. Rev. 1954, 95, 359–369. (17) Braslau, A.; Pershan, P.; Swislow, G.; Ocko, B.; Als-Nielsen, J. Phys. Rev. A 1988, 38, 2457–2470.
15946 DOI: 10.1021/la102882j
low for a significant contribution to the scattering signal. The small layer thickness and the fact that the layer has no internal structure (i.e., signatures of the headgroup and tail region usually observed for a well-ordered compressed Langmuir film) indicate that the nanoparticles have partially disconnected their lipid shell that now forms an uncompressed gaseous Langmuir film of lauric acid on the water surface. Nanoparticles in the water subphase were observed at the end of the experiment, showing the disconnection of the lipids. In this state, the formed film can be removed from the surface, thus allowing the recovery of the water surface tension for several minutes. Afterwards, lipids appear again at the surface. Thus, nanoparticles in the water phase continue to discharge lipids. The reflectivity of the compressed film shows a strong oscillation at low qz values. This is an indication of the formation of a thick film at the liquid-gas interface with a high electron density compared to that of water. The reflectivity could be refined using an electron density model consisting of a Langmuir layer in a solid phase, modeled by a headgroup and a lipid tail, and one layer for the nanoparticles attached below the Langmuir layer (Figure 1). To refine the nanoparticle layer, an iterative algorithm was applied that modulates the electron density profile stochastically18 using a total sample depth of 400 A˚ and a fwhm of the displacement of 30 A˚ in the beginning and 10 A˚ at the end of the iterations. The asymmetric shape of the adsorbed layer is due to an asymmetric particle size distribution within the layer, which will be discussed below. The total thickness of the refined Langmuir layer of dla = (20.1 ( 0.2) A˚ indicates an upright configuration of the lipids.19 Thus, the gaseous Langmuir layer observed at π = 0 mN/m has been compressed into the solid phase. In contrast to hydrophilic nanoparticles that penetrate the Langmuir film,15,20 a clear separation between nanoparticles and the Langmuir film can be observed. This finding shows that the particles lose their hydrophobic shell on their water-allocated side whereas they perfectly integrate their lipid residues on the top side into the formed Langmuir film, as illustrated in Figure 3, left. Because nanoparticles that lose their coating completely would agglomerate and drop down to the trough bottom, the pure adsorption of bare particles at the interface can excluded for the majority of particles. Thus, the previously hydrophobic nanoparticles have (18) Sanyal, M. K.; Hazra, S.; Basu, J. K.; Datta, A. Phys. Rev. B 1998, 58, R4258. (19) Zaleskaa, A.; Nalaskowskib, A.; J., H.; Millerb, J. Appl. Catal. B 2009, 88, 407–412. (20) Degen, P.; Paulus, M.; Maas, M.; Kahner, R.; Schmacke, S.; Struth, B.; Tolan, M.; Rehage, H. Langmuir 2008, 24, 12958–12962.
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Figure 2. (Left) Electron density profile of the liquid-gas interface at π = 6 mN/m. The gray line represents the density profile obtained by a refinement of the reflectivity data, and the dashed line represents the density profile calculated using eq 2. (Right) Particle size distributions obtained by the analysis of X-ray reflectivity measurements (-) and dynamic light scattering (9).
Figure 3. (Left) Sketch of the arrangement of hydrophobic nanoparticles that partially lose their hydrophobic shell. The detached lipids form a Langmuir layer, and the residue was integrated into the formed film. (Right) AFM image of the film transferred on a silicon wafer.
automatically changed to particles with a hydrophobic and a hydrophilic side, so-called Janus beads.6-8 An AFM image of a nanoparticle film transferred onto a silicon wafer is shown in Figure 3 (right). It is obvious that the nanoparticles arrange in small islands and do not form a laterally homogeneous film. The formation of small islands of nanoparticles is also visible if the film is transferred at a surface pressure of π = 0 mN/m, but the surface coverage at 0 mN/m is lower. Thus, the concentration within the film scales with the surface pressure. For further analysis of the nanoparticle film profile, a logarithmic particle size distribution defined by 2 lnðri =μÞ -1=2 lnðΔμÞ u e ð1Þ ξðΔμ, μ, ri Þ ¼ pffiffiffiffiffiffi 2π lnðΔμÞri was used, with the most probable particle radius μ, the scaling amplitude u, and the parameter Δμ that defines the width of the particle size distribution. The vertical electron density profile A(z) of the polydisperse sphere distribution was calculated via AðzÞ ¼
X ½ð2πri z - πz2 Þξðri Þ
ð2Þ
i
with z e ri. A refinement of eq 2 to the electron density profile below the Langmuir film yields μ = (49 ( 0.5) A˚. The electron density profile obtained by the refinement of eq 2 is shown in Figure 2 on the left side as a dashed black line together with the electron density profile obtained by the refinement of the X-ray reflectivity data (gray line). Obviously, a good agreement was reached. The resulting particle size distribution is displayed in Langmuir 2010, 26(20), 15945–15947
Figure 2 on the right side together with the distribution obtained by dynamic light scattering. It is clearly visible that the maximum and the tail for R g μ of the two particle size distributions obtained by X-ray reflectivity and DLS agree very well whereas differences are observed in the lower part of the distribution functions. Here, the particle size distribution derived from the X-ray reflectivity measurement predicts a greater number of small particles. This difference can be the result of the high spatial resolution of the X-ray reflectivity technique or might be the result of deviations of the nanoparticles from spherical particle shape. The choice of a logarithmic particle size distribution function can be justified by the nanoparticle formation process that is based on the agglomeration of particles within the solution. The formation of large particles at the cost of small ones causes the asymmetric shape of the particle size distribution. In summary, the presented study shows that the nanoparticles’ ability to detach the hydrophobic shell can be used for the synthesis of well-ordered nanoparticle films. The layer system shows a clear separation between the Fe3O4 nanoparticles and the Langmuir film, which was not observed using hydrophilic nanoparticles.15,20 The partial detachment of the lipids from the nanoparticles leads to the formation of a layer of self-organized Janus beads at the liquid-gas interface exhibiting a hydrophobic and a hydrophilic side. The logarithmic particle size distribution function of the nanoparticles forming the adsorbed film was congruent with the distribution in the bulk liquid as determined by dynamic light scattering. Acknowledgment. We acknowledge the DESY member of the Helmholz Zentrum for providing synchrotron radiation. T.B. and S.T. acknowledge the Forschungsschule Forschung mit Synchrotronstrahlung in den Nano- und Biowissenschaften for financial support. DOI: 10.1021/la102882j
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